The addition of titanium to steel in general, and to low-alloy steels in particular, is a well-known means for controlling the high temperature austenitic grain, for example in the heat-affected zones during welding, and, where appropriate, to harden the structure obtained after heat or thermomechanical treatment. In order to manufacture these steels, liquid steel free of titanium is smelted, the liquid steel is vigorously deoxidized, for example by the addition of aluminum, and then blocks of ferro-titanium are added which progressively dissolve.
While the ferro-titanium blocks are dissolving, the titanium reacts with the nitrogen present in the steel and forms relatively coarse precipitates having a size on the order of 1 .mu.m or more. It is these precipitates which, after the steel has solidified, prevent austenitic grain growth. However, these precipitates have several drawbacks because they are sharp-angled, relatively coarse and consequently relatively few in number; their effect on the hardening and refining of the microstructure is limited and they degrade the fracture toughness of the steel.
It has been proposed, especially in EP 0,177,851, to manufacture low alloy titanium steels, having a very low aluminum content, in which the titanium is in the form of oxides. These oxides act as preferred sites for nucleation of ferrite during transformations from austenite to ferrite/pearlite; they thus result in a refinement of the ferrito-pearlistic structures, which improves the toughness, especially of welded joints, very considerably. However, this technique has several drawbacks: it requires a very low aluminum content, which is prejudicial to controlling the austenitic grain during heat treatments and requires a very short casting time and a very short solidification time, which complicates manufacture, and it only has an effect on the ferrito-pearlistic structures.